Multistatic surveillance systems, in a known manner, have certain specific features which differentiate them in terms of design, production, deployment and maintenance from monostatic systems.
Amongst these specific features it is possible in particular to note the fact that such a system usually uses the radioelectric transmissions produced by several distinct transmitters preferably dispersed around the zone to be monitored or else inside this zone, these transmitters being cooperating items of equipment forming part of the system, or non-cooperating items of equipment the transmissions of which have a primary finality other than use for detection purposes. These can be broadcasting transmitters for example.
On the subject of specific features, it is also possible to note that, in such a system, there are usually several items of equipment responsible for receiving the radioelectric signals, these items of equipment being dispersed in the zone to be monitored.
It is also possible to note on the subject of the specific features that, in such a system, which like any detection system comprises a command and decision unit, the link between the command unit and the various transmitters and receivers requires the installation of communication means making it possible to both control the various elements from the command unit and to retrieve and exploit the information generated by these elements, in particular the information supplied by the various receivers relating to the objects detected in the monitored zone.
Consequently, when such a system is used, various problems arise, amongst which it is possible to cite:
problems associated with the spectrum of the transmitted signals and with the determination of the source of the received signals,
problems associated with using communication means between the various elements of the system and the command unit,
problems associated with the tolerance of the system to faults,
problems associated with the capacity of the system to adapt to variations in its environment.
The problems associated with the spectrum of the signals transmitted and to the determination of the source of the signals received originate notably from the fact that a given receiver can exploit the signals that it receives only if it can identify and locate the source that has transmitted the radioelectric signal in question. This results in practice in the following two requirements:
a) the receiver has knowledge of the relative position of each transmitter that can be the source of a received signal,
b) the receiver can recognize at any time which of the transmitters is the source of the signal that it receives.
Usually, and in particular if the transmitters used by the system are transmitters dedicated to this usage, these requirements are lifted by allocating a particular frequency band to each transmitter. Therefore since each of the receivers knows the spectral distribution of the transmissions, a given receiver can determine which transmitter is the source of the received signal. Consequently, the (bistatic) position of an object reflecting this signal can be determined in a known manner by measuring the time lag that separates the reception of the signal reflected by the object from the reception of the signal received directly from the transmitter.
This solution has the advantage of being simple to apply in principle, each transmitter then having a frequency band that is specific to it. However, if the system uses a considerable number of transmitters, each transmitter has a limited bandwidth for transmitting which, because the total frequency band allocated to the system is necessarily limited, may be insufficient to satisfy the resolution requirements imposed on the detection system.
The problems associated with using communication means between the various elements of the system and the command unit are, for their part, usually associated with the compromise to be made between the cost of producing and using the chosen means and the requirement to produce a multistatic detection system having a given flexibility. In other words, an installation comprising communication means using wire links will, for example, be less costly to integrate into the system in question than an installation comprising radio communication means. Specifically the installation of radio-link means requires in practice that each element of the system, transmitter, receiver and command unit, is provided with appropriate radio means.
On the other hand, an installation comprising communication means using wire links will, by its nature, be less easy to deploy over a wide monitoring area. It will also be more likely to see its effectiveness degraded, and even to break down, because of the intentional or unintentional breakdown of one or more wire links, which breakdown may occur during the period of operation of the system. Moreover, a wire communication structure seems in practice to be not very well suited to the deployment of a mobile detection system.
The problems associated with the capacity of the system to adapt to variations in its environment relate in particular to the capacity of the detection system to modify the waveforms transmitted by the various transmission means according to variations of the environment in which the surveillance zone is situated. This capacity is in practice limited by the bandwidth allocated to each transmitter, so that this capacity to adapt is usually limited.
In an attempt to limit the effects of the various requirements cited above, the user usually resorts to using several specific solutions, each solution being intended to solve a particular problem. Consequently the addition of means materializing these solutions makes the use of the system more complex and the system itself more costly.